1. Field of the Invention
The present invention generally relates to the recovery of soda ash from aqueous brines. More particularly, the present invention relates to a process for obtaining soda ash from sodium bicarbonate containing solution mined brines.
2. State of the Art
Dry mining and solution mining techniques have been used to recover soda ash from sodium bicarbonate containing ores such as trona and nahcolite. One of the most common processes for producing soda ash from dry mined sodium bicarbonate containing ores is by the monohydrate route, wherein sodium carbonate monohydrate crystals are formed from an aqueous solution containing sodium carbonate. In such a process as practiced at Green River, Wyo., dry mined trona ore is calcined to form a crude soda ash product which is dissolved in water and then clarified and filtered to form an aqueous solution. The aqueous solution is introduced to a sodium carbonate monohydrate crystallizer where water is evaporated to form a slurry of sodium carbonate monohydrate crystals in a mother liquor. The sodium carbonate monohydrate crystals are separated from the slurry and calcined to form a dense soda ash product. Mother liquor separated from the slurry may be recycled to the sodium carbonate monohydrate crystallizer. This type of direct processing of brine through a monohydrate process has generally not been practiced with solution mined brines containing sufficient sodium bicarbonate.
U.S. Pat. No. 4,869,882 to Dome et al. describes a process for treating waste pond solutions containing significant quantities of sodium carbonate and sodium bicarbonate as well as silicates, chlorides and sulfates. Dome neutralizes, clarifies, heats, cools and evaporates such solutions to obtain a liquor having about 17-24% Na2CO3, from which sodium carbonate decahydrate crystals are produced by cooling. Dome notes that monohydrate crystals cannot be directly obtained from solutions containing significant sulfate contamination since burkeite crystals (Na2CO3.2Na2SO4) are also formed, thereby producing impure monohydrate crystals. Dome indicates this problem is avoided by first producing decahydrate crystals.
In U.S. Pat. No. 4,034,617 to Kuo, a process for treating monohydrate mother liquor in a decahydrate crystallizer is described. Kuo""s monohydrate process starts with dry mined, calcined trona which is dissolved in a carbonate mother liquor to form the monohydrate feed stream which contains no significant quantity of sodium bicarbonate. A portion of the monohydrate crystallizer mother liquor, which typically would be purged, is instead fed to a decahydrate crystallizer where decahydrate crystals are formed. As reported in the patent, the decahydrate crystals are substantially free of contaminants. The decahydrate crystals are then heated to melt the crystals to form a hot monohydrate slurry from which monohydrate crystals are separated and sent to a dryer to be converted to soda ash, along with monohydrate crystals from the main monohydrate crystallizer.
A process for recovering soda ash from a sodium carbonate and sodium bicarbonate brine is described in U.S. Pat. No. 5,283,054 to Copenhafer et al. In this process, a sodium bicarbonate and sodium carbonate containing brine derived by solution mining is heated and steam stripped to decompose sodium bicarbonate to sodium carbonate, water and carbon dioxide. The stripped brine is concentrated by evaporation and neutralized with caustic soda to further reduce the sodium bicarbonate concentration in the brine to a very minimal amount to form a brine concentration suitable for crystallization of sodium carbonate decahydrate crystals therefrom. This brine is introduced to a sodium carbonate decahydrate crystallizer where it is cooled to form a slurry of sodium carbonate decahydrate crystals and a mother liquor.
The sodium carbonate decahydrate crystals are separated from the mother liquor, melted and diluted with water to make a nominal 30% by weight sodium carbonate solution. Additional caustic soda may also be added to the solution to further reduce sodium bicarbonate concentrations therein. The solution is crystallized by evaporation of water in a monohydrate crystallizer to form a slurry of sodium carbonate monohydrate crystals in a mother liquor. These monohydrate crystals are separated from the slurry and processed into a dense soda ash product by calcination. The ""054 process requires a very large decahydrate crystallizer and significant cooling capability, which requires significant energy. The ""054 process may be used where a solution mine brine has a significant impurity content.
Although the available processes may be used to produce soda ash from dry mined or solution mined ore, improvements in the production processes are always desired. Recent improvements in bicarbonate and/or carbonate brines from sodium containing ores, especially with regard to brines resulting from in situ dissolution of such ores, have been directed towards initially forming brines which are suitable for crystallization to form sodium carbonate decahydrate crystals.
The present invention generally relates to the recovery of sodium values from aqueous brines containing a minimum of impurities. More particularly, the present invention relates to a process for obtaining soda ash from sodium bicarbonate containing solution mined brines via initially forming a brine suitable for feeding to a monohydrate crystallizer.
The invention, more specifically, provides a process for optimizing sodium carbonate monohydrate recovery from a mine brine containing significant quantities of sodium bicarbonate and minimal impurities by evaporating and/or stripping such a mine brine to concentrate sodium values in the mine brine and convert at least a portion of the remaining sodium bicarbonate therein to sodium carbonate to form a concentrated brine. At least a portion of remaining sodium bicarbonate in the concentrated brine is neutralized with hydroxide to form additional sodium carbonate, thereby forming a crystallizable solution of a composition from which monohydrate crystals of sodium carbonate will form upon evaporation of water from the crystallizable solution in a monohydrate crystallizer at a temperature of from about 35xc2x0 C. to about 109xc2x0 C. A slurry is formed in the crystallizer comprising sodium carbonate monohydrate crystals and a mother liquor containing dissolved sodium carbonate in a concentration suitable, optionally, as feed to a sodium carbonate decahydrate crystallizer.
The sodium carbonate monohydrate crystals are separated from the mother liquor to recover the mother liquor apart from the sodium carbonate monohydrate crystals. In one embodiment of the invention, at least a portion of the mother liquor is fed to a sodium carbonate decahydrate crystallizer to crystallize sodium carbonate decahydrate crystals as a slurry in a decahydrate mother liquor which is separated from the sodium carbonate decahydrate crystals. The decahydrate crystals are recycled to the monohydrate crystallizer. Either of the mother liquors may be recycled to form part of the aqueous solvent provided that the level of impurities in the resulting mine brine is controlled within predetermined levels.
The initial recovery of monohydrate crystals, especially when accompanied by secondary recovery of decahydrate crystals which are recycled to the monohydrate crystallizer, optimizes recovery of monohydrate crystals in a manner which is energy and equipment efficient. Coprecipitation of burkeite crystals is generally avoided by utilizing a monohydrate crystallizer feed stream which has a relatively low sulfate content, accompanied by a relatively low level of sodium chloride. Burkeite is a double salt of sodium carbonate and sodium sulfate (Na2CO3.2Na2SO4). Increasing NaCl concentration decreases the tolerable level of Na2SO4 to prevent burkeite formation. Cocrystallization of burkeite can result in unacceptable high levels of sulfate in the final soda ash product.